Top-loaded liquid filter housing module designs, like those used in engine lubrication systems, which use a replaceable filter element removably disposed within a permanent housing and lid assembly, can create a “trapped air” zone above the filter element. These designs can also include a bypass valve on a centertube of the housing. The bypass valve is intended to open during an unusual “cold start” condition when low oil temperatures (and very high oil viscosity) cause such high pressure drop across the filter media that the pleated element structure could be collapsed, thereby necessitating the bypass valve as a “pressure relief” to avoid filter damage. The bypass valve may also open if the filter itself becomes completely plugged with debris, sludge, etc. and creates excessive pressure drop even during normal hot oil (low viscosity) conditions, in which case it is much more important to supply oil to the engine than to provide the continued full-filtration of oil.
During oil change servicing and replacing the filter element by removing the housing lid, removing the filter element (which also allows oil to drain from the housing through a drain feature), inserting a new replacement filter element, and re-attaching the sealed lid, the housing is left in a condition where it is filled mainly with air. Upon starting the engine, the lube pump immediately supplies oil to the housing, filling it from the “bottom up” and pushing the air out of the housing through the filter media and purging it through the lube system passages and ultimately back to the crankcase sump.
However, the air trapped at the top of housing cannot be purged, since it is above the endcap and filter media, and the bypass valve is sealingly engaged with the endcap. This trapped air may cause slightly slower delivery of full operating pressure to critical downstream components such as turbo bearings, main bearings, valve train, etc. due to the compressibility of air acting as an “accumulator” each time the engine is started. In addition, the trapped air, and its compressibility, acting as an accumulator or “air spring,” could also cause system pressure/flow pulsation/oscillation/instability problems due to the complexity of the lube fluid flow circuit containing pressure regulation devices, flow path splits, valves, etc.